1. Field of the Invention
The present invention relates to a facility for electrical function checking of wiring matrices through a plurality of gas discharge channels.
2. Description of the Prior Art
In automatic test units and test adaptors for use with both equipped and unequipped circuit boards, as well as for use with soldered or crimped wiring matrices, resilient test probes are usually used to contact the selected measuring locations. The resilient test probes are arranged in a grid pattern of a wiring matrix to be checked and are secured by spring sleeves which are pressed into a carrier plate and into which the test probes are inserted. Test probes are usually selected to accommodate the smallest spacing of the test locations relative to one another as well as to accommodate the current load across the diameter of the resilient test probes. The lower limit for the diameter dimension of the test probes is cited as 0.69 mm in the publication Elektronik Produktion & Prueftechnik, November 1979, pages 472 and 473.
Tests which can be performed include conductivity and insulation measurements between test locations of printed circuit boards according to the grid points by the known testing devices. Since the resilient test probes provided for contacting the test locations must be arranged in the grid of the printed circuit board, it is becoming increasingly difficult to overcome the difficulties for the now decreasing grid dimensions and increasing areas of printed circuit boards. Thus, with an arrangement of resilient test probes in grid dimensions of less than 1 mm, it is now difficult to precisely provide a reliable mechanical contact with the test locations. The test locations can now amount to over 100,000 on a single circuit board or wiring matrix so that the number of leads and switch elements required increases accordingly causing considerable apparatus-oriented expenditure and correspondingly high costs. Moreover, as the number of test locations increases, the probability of contacting all of the test locations of a printed circuit board decreases noticeably.
In European Patent EP-A No. 0 102 565, a facility for electrical function checking of wiring matrices is disclosed wherein standard ohmic contacting of the test locations is replaced by a non-contacting ionic contact through gas discharge paths. A carrier plate is placed on the wiring matrices and has a plurality of gas discharge channels provided with electrodes therein. The gas discharge channels are arranged in a grid of the wiring matrices and are open toward the test locations. When two selected test locations are connected to one another in an electrically conductive fashion, such as by an interconnect or land, then the respective gas discharge channels form two gas discharge paths connected in series which can be ignited by applying an adequately high voltage to the electrodes.
The ignition of the gas discharges ensues to provide a current flow which can be interpreted for test purposes. When the ignition of the gas discharges is suppressed or when the current flow is too low after an ignition has occurred, then it can be concluded that the electrically conductive connection has been interrupted or that the electrically conductive connection did not exist at all from the very outset between the selected test locations. When an AC signal superimposed over the ignition voltage is applied to the electrodes, then the phase of the resulting current change can be compared to the phase of the applied AC voltage to determine the resistance of the connection between the selected test locations.
The known apparatus, thus, enables conductivity and insulation measurements of extremely high reliability since ohmic contacts are avoided. Wiring matrices that have, for example, small grid dimensions of the test locations of down to 0.1 mm can reliably be checked through ionic contacting of the test locations by gas discharge channels since such channels are realizable in extremely small dimensions. When a high number of test locations are present on a circuit board which is being checked, however, problems attributable to the numerous leads and switch elements for connecting the electrodes of the gas discharge channels still exist.